Photoelectrochemical (PEC) water splitting for solar hydrogen production has attracted extensive interest in the last few decades. Titanium dioxide (TiO2) has been extensively investigated as a photoanode for photoelectrochemical (PEC) water splitting because of its favorable band-edge positions, strong optical absorption, superior chemical stability and photocorrosion resistance, and low cost. However, the STH efficiency of TiO2 is substantially limited by its large band gap energy and usually fast electron-hole recombination due to a high density of trap states. An enormous amount of research has been focused on enhancing the visible light absorption of large band gap metal oxides. For instance, sensitization with small band gap semiconductors and/or band gap narrowing via elemental doping are two versatile approaches shown to improve the conversion efficiency of metal oxide photoelectrodes by modifying their optical absorption coefficient and wavelength. On the other hand, it is equally important to fundamentally improve the morphology and electronic structure of TiO2 for effective separation and transportation of photoexcited charge carriers. It has been predicted that a maximum photoconversion efficiency of 2.25% can be achieved by rutile TiO2 with an optical band gap of 3.0 eV, at 100 mW/cm2 AM 1.5 global illumination. Yet, the reported photocurrent densities and photoconversion efficiencies of TiO2 photoanodes are substantially lower than the theoretical limit.
Additionally, WO3 as a photoanode material has attracted extensive attention due to its favorable bandgap. However, photoelectrochemical instability is a major hurdle for the WO3 photoanode, as the peroxo-species generated in water oxidation cause photocorrosion of WO3. An effective approach to stabilize WO3 is to deposit a layer of oxygen evolution catalyst to reduce oxygen evolution overpotential and therefore suppress the formation of peroxo species. However, a thick catalyst layer will sacrifice the photoactivity of WO3 by blocking the light penetration.
Further, zinc oxide (ZnO) has been extensively studied for photocatalytic hydrogen evolution because it has favorable band-edge positions that straddle the redox potential of water photoelectrolysis, and it is of low cost and environmentally friendly. Nevertheless, the efficiency of ZnO for photocatalytic hydrogen evolution is limited (200-2000 μmol h−1 g−1) by its wide band-gap and rapid recombination of photo-generated carriers. Moreover, the excessive aggregation of ZnO powder photocatalysts in water causes substantial reduction of active surface area, and thus decreases the catalytic performance and utilization of ZnO.
What is needed is a treatment that can significantly enhance the photoconversion efficiency of BiVO4, TiO2, WO3 and ZnO materials by improving their donor density and electrical conductivity.